The typical losses in light throughput associated with a length of solid light pipe include the following five factors: (1) absorption in the light-transport material, (2) light scattering due to impurities present in the light-transport material, (3) light scattering at the core-cladding interface, (4) light lost due to bends in the light pipe, and (5) light lost at the input and output ends of the light pipe due to Fresnel reflections. The first three factors (1)–(3) are associated with the light-pipe materials: how such materials interact with the other components of the light pipe, and how the light pipe is bent for routing purposes. These losses can be expressed as an “attenuation number,” which is a percentage of lost light per foot of light pipe, with such losses increasing as the length of the light pipe increases. The losses associated with how the light pipe is bent are dependent on the light pipe diameter, the radius of the bend, and the angle of light that the light pipe can transmit without a loss in light.
The losses due to Fresnel reflections are independent of the material of the light pipe and are constant irrespective of the length of the light pipe. The losses associated with Fresnel reflections are on the order of 4% per surface. A typical method for regaining this light lost on surfaces with a large area, such as desktop computer monitor screens, is to apply an anti-reflective (AR) thin film onto the surface. Typically, this preserves 2–3% of the light at each surface that would be otherwise lost to Fresnel reflections.
Most commercial AR coaters apply an AR coating to a large thin sheet, or to a thin film roll as it is rolled through the coater and re-rolled at the output. Such AR coaters are not designed to handle the much different geometry of the end-faces of light-transmitting cores of light pipes, which may typically be from 3 mm to 25 mm in diameter, and whose length typically varies from a few up to 30 meters. In short, commercially available coaters are designed to coat large-area sheets or films, not the end-faces of long light pipes.
Another problem with the commercial AR-coaters for coating thin films is that the operating temperature of these systems is generally around 100 degrees C. The polymer core of the light pipe at this temperature will survive; however, the polymer becomes soft and swells, so that the core face is no longer flat, but bulges in a rounded fashion. This bulging of the polymer fiber core would result in the AR coating cracking and flaking off when the core cools and becomes flat again. Other commercial AR coaters capable of coating the end-face of a light pipe—such as those used in e-beam evaporation machines used to coat the end-faces of glass rods up to 25 mm in diameter—apply coatings at an elevated temperature such as more than 250 degrees Centigrade. Such temperatures would damage plastic light pipes comprising a polymer, so the technique would not be successful even if AR-coating machines were modified to accommodate the much different geometries of light pipes.
Additionally, for any AR coating, or AR-coated substrate, for a solid light pipe, the following features, among others, would be desired: (1) resistance to heat and light encountered in normal use of light pipes, (2) high optical clarity, and (3) resistance to cracking in normal use of the light pipe.
It would be desirable to provide an arrangement for supplying a suitable AR coating at one or both end-faces of a solid, polymer-based light pipe. This would reduce Fresnel-reflection losses, increasing light throughput in the light pipes.